SPID#: 42 This study will use the rhesus monkey to develop gene therapy approaches for diseases involving mitochondrial DNA (mtDNA). The target diseases are Chronic Progressive External Ophthalmoplegia (CPEO) and the Kearn's Sayre Syndrome, both diseases of extraocular muscles and both due to single gene mutations in mtDNA Clinical studies of these syndromes are underway at the Gene Therapy Suite of the Emory Clinical Research Center (CRC). At Yerkes, myoblast cultures from rhesus monkey extraocular muscles have been established for optimizing cybrid transfer. Following transfection with beta-galactosidase-extraocular muscle. Recombinant cells can be detected by beta-galactosidaseosidase staining. This model system has the advantage of providing a small muscle where sufficient transfected cells can be reintroduced to alter function. When the optimal conditions for transfection and reimplantation are defined in the monkey, the same approach will be attempted in the patients studied in the CRC.